Conventional bone prostheses or implants that are known in the art generally include a metal portion constructed of steel or titanium inserted in and fixedly attached to a bony portion of a patient's body. For example, conventional implants for stems of articulating joints or for nail-like implants used for intramedullary fixation to support bones during healing of fractures are constructed of metal. The nail-like implants are usually, but not always, removed when healing is complete.
Current implant technology may be divided into two broad categories: implants which require reaming of an inner canal in the bone before insertion of the implant, generally typical of relatively thick implants; and implants which do not require reaming of a bone canal, generally typical of relatively thin implants. Thin implants are easier to insert and they enable better nourishment of the bone and faster healing. However, thin implants provide a less stable support structure for the bone.
Several problems are associated with conventional bone implants due to a mismatch between materials properties of the bone and the metal implant. For example, contact between the metal implant and the bone may cause fretting wear of the bone. Also, a difference in materials properties such as Young's modulus and thermal expansion coefficient between the metal implant and the bone may result in poor anchoring of the metal implant to the bone, which may cause discomfort to the patient, especially during weather changes. Furthermore, conventional metal implants provide virtually no shock absorption or damping.
It is generally known that a bone grows or generates new bone tissue according to the level of stress to which it is subjected within an identified range of stress levels that is less than or equal to a certain maximum stress level but greater than or equal to a certain minimum stress level. One problem with conventional metal implants is that they tend to distribute stress unevenly to the surrounding bone, with some surrounding bone areas receiving excessive stress levels and other surrounding bone areas receiving less than optimal stress levels. In extreme cases where the amount of stress imparted to a surrounding bone area is too low, the conventional metal implant may contribute to atrophy or degeneration of the area because of lack of use. This, in turn, may lead to bone recession and loosening of the anchoring of the metal implant, thus creating an undesirable gap between the bone and the metal implant.
Another problem with conventional metal implants is their tendency to interfere with diagnostic techniques such as magnetic resonance imaging (MRI) and related tomographic techniques. Metal implants produce shadows, or image artifacts, because the metal material of the implants tends to interfere with the magnetic characteristics of the implant region, thus distorting the magnetic image of that region. These shadows reduce the effectiveness of magnetic resonance techniques, which are important and widely used diagnostic techniques that have even been used during surgical operations to produce real-time images with simultaneous multiple image sections. Therefore, the use of non-metal implants would be advantageous because such implants are expected to reduce the presence of these undesirable shadows, or image artifacts, to a level that will preserve the effectiveness of MRI-type images for diagnosis.
Yet another drawback of conventional implants is their inability to conform to the three-dimensionally curved surfaces of a bone canal, thus reducing their effectiveness in inducing or promoting continuous bone growth or bone reinforcement for strengthening the bone.
In order to overcome the aforementioned problems, a number of implants with resilient portions have been proposed in International Application No. PCT/IL96/00098, which was filed Sep. 4, 1996 and invented by the inventor of the present invention, the disclosure of which is incorporated herein by reference. This PCT application discloses various resilient joint prostheses and bone implants that provide shock absorption and promote bone development and growth after implantation. In addition, the PCT application discloses bone implants that are sufficiently flexible that they deform to adapt to various and changing curvatures of the bone.